Bipolar Disorder (BD) is a common, severe, chronic and often life-threatening illness. Suicide is the cause of death in 10-20% of individuals with BD. The discovery of lithium's efficacy as a mood-stabilizing agent has since revolutionized the treatment of patients with BD. However, approximately 50% of patients do not respond fully to lithium, and the biochemical basis for lithium's antimanic and mood-stabilizing actions remains to be fully elucidated. Elucidation of the mechanism(s) by which lithium stabilizes an underlying dysregulation of limbic and limbic-associated function also offers the potential to delineate the underlying pathophysiology of BD; however, a major problem inherent in neuropharmacologic research is the difficulty in attributing therapeutic relevance to any observed biochemical finding. One powerful approach is to identify common biochemical targets, which are modified by drugs belonging to the same therapeutic class (e.g. mood-stabilizing agents) but possessing distinct chemical structures when administered in a "therapeutically relevant" paradigm (i.e., effects which are observed upon chronic drug administration, and yet persist beyond abrupt drug discontinuation). In this context, it is noteworthy that both valproic acid (VPA) and lithium, with different chemical structures, belong to the same therapeutic class and cause considerable inhibition of protein kinase C (PKC). The PKC signaling pathway is clearly a target for the actions of two structurally highly dissimilar antimanic agents -- lithium and VPA. Do these effects of lithium and VPA on PKC signaling have any clinical relevance? There is thus a clear need to investigate the potential efficacy of a direct PKC inhibitor in the treatment of acute mania. There is currently only one relatively selective PKC inhibitor available for human use- Tamoxifen. Tamoxifen a synthetic nonsteroidal antiestrogen, has been widely used in the treatment of breast cancer. Tamoxifen's potent inhibitory effects on PKC are striking. Recently, our group conducted the first open-label study with Tamoxifen in acute mania. In this study, Tamoxifen resulted in a significant decrease in manic symptoms within a short period of time (3-7 days). The overarching goal of this proposal is to test the hypothesis that PKC inhibition is part of the mechanism of the therapeutic effect of mood stabilizing drugs. The proposal derives from and builds on our published open-label study of Tamoxifen in acute mania (Bebchuk et al., 2000). However, the efficacy of Tamoxifen monotherapy in acute mania has only been reported in an open-label study and has not yet been evaluated in a randomized, double blind, placebo-controlled study. Male or female patients, ages 18 to 65, with a diagnosis of bipolar I disorder manic or mixed (with or without psychotic features), will be randomized to double-blind treatment to receive either Tamoxifen (20-140 mg/day) or placebo, for a period of 2 weeks. Approximately 50 patients with acute mania will be enrolled in the study. Biochemical measures will be obtained during the study.